A vinegar smell might be like ammonia and is often associated with urine, to be honest. :>) Foul smelling is different than that and that is usually bacterial when you have that. Our discharge changes with our hormones. It probably changes in minute or even more noticeable ways as you go through your cycle. The two times to worry are if the A. smell really bad, thin greyish watery discharge (bacterial vaginosis) or B. itches (often clumpy but not always).
If your have pain though, you should check on that. It may not be related to your period or reproductive tract at all but no one should have gi or pain issues for three straight weeks! So, do speak to your doctor! good luck
If you have discharge that smells, it's always a good idea to have a doctor check that out- it can be a sign of an infection and should be checked into just in case.